Process for the manufacture of aromatic amines by gas-phase hydrogenation and catalyst useful thereof

ABSTRACT

Aromatic amines are produced by catalytic hydrogenation of nitroaromatic compounds in the gas phase. The catalyst includes palladium and lead on graphite or on a graphite-containing coke support. From 30 to 6000 equivalents of hydrogen for each equivalent of nitro groups are contacted with the catalyst during this process.

This application is a division of application Ser. No. 08/660,622 filedJun. 6, 1996, now U.S. Pat. No. 5,679,858.

BACKGROUND OF THE INVENTION

The present invention relates to an improved process for the manufactureof aromatic amines by catalytic hydrogenation of nitroaromatics in thegas phase and to a new catalyst useful in this process.

Anilines are important intermediate products for the manufacture ofdyestuffs, polyurethanes and plant-protection products.

Various methods for hydrogenation of nitrobenzene and othernitroaromatics are known. Due to the large enthalpy of reaction releasedduring these known processes, all are carried out in reactors havingintegrated heat-carrier systems. For example, hydrogenation in liquidphase on a suspended catalyst (e.g., Pd catalyst) is described in EP476,404. Hydrogenation in the gas phase on a fluidized solid catalyst isdisclosed, for example, in U.S. Pat. No. 3,136,818. Hydrogenation in thegas phase on a stationary catalyst (e.g., a supported Pd catalyst) isdescribed in DE-A 2,244,401; 2,849,002; and 4,039,026.

In DE-A 2,244,401 and 2,849,002, Pd catalysts on aluminum oxide supportsthat can be operated as stationary catalyst beds in heat-exchanger tubesunder normal pressure at loadings of less than 1 g nitrobenzene (Nbz)/mlcatalyst h with low hydrogen/nitrobenzene ratios are described. Between6 and 11 moles of hydrogen per mole of nitrobenzene are fed into thereactor. These catalysts must be regenerated approximately every 1000hours. Each of the described processes is operated in a Gas Hourly SpaceVelocity (GHSV) range between 600 and 900 h⁻¹.

In DE-A 4,039,026, Pd catalysts on graphitic supports are described. Theprocess described in this disclosure is carried out under conditionssimilar to those used for processes in which Pd catalysts on aluminumoxide are used. At loadings clearly below 1 g (Nbz)/ml catalyst h and ahydrogen/nitrobenzene ratio of 14-26 moles to 1 mole, the catalysts showincomplete conversion. Between 1000 and 4000 ppm nitrobenzene, relativeto aniline formed, are found in the condensate. The selectivitiesrelative to aniline vary between 99.1 and 99.6%. The process isdescribed for a GHSV range between about 2000 and 3150 h⁻¹.

Both an increase of the nitroaromatic loading and a raising of thehydrogen/nitroaromatic ratio raise the volumetric flow rate through thecatalyst bed thus reducing the residence time on the catalyst. Bothmeasures should therefore lead to an increase of the nitroaromaticbreakthrough (i.e., incomplete conversion).

A general measure of this gas flow through the catalyst bed is the GasHourly Space Velocity (GHSV), quoted in the unit h⁻¹.

However, even small amounts of nitroaromatics in aromatic amines lead todistinct discoloration of the otherwise colorless aromatic amine and aretherefore undesirable. Removal of the nitroaromatics by distillation isdemanding, both with respect to the apparatus and to the amount ofenergy consumed.

In each of these process variants, the large heat of reaction generatedmust be withdrawn from an industrial reactor via a complicated heatcarrier system.

Hydrogenation processes in the gas phase with simple adiabatic catalystbeds are particularly economical because simple apparatus without anintegrated heat exchanger system may be used. However, the largeexothermicity of nitro group hydrogenation means that specialrequirements for the catalyst must be met. The catalyst must hydrogenateselectively over a wide temperature range. In the adiabatic process, aheat carrier (in hydrogenation processes this carrier is usuallyhydrogen) is also admixed with the educt mixture which leads to veryshort residence times (i.e., large GHSV's). The catalyst must therefore,in addition to its selectivity over a wide temperature range, be veryactive in order to obtain a complete nitrobenzene conversion, even atlow nitrobenzene loadings.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a catalyst with ahigher loadability than the known catalysts, i.e., a catalyst whichshows complete conversion at large GHSVs and which can be used in simplereactor structures.

It is also an object of the present invention to provide a process forproducing aromatic amines which achieves continuously high selectivitiesfrom the startup of the catalyst to its deactivation, so that anexpensive working-up by distillation of the aromatic amine condensatewith elaborate apparatus is not necessary.

It is another object of the present invention to provide a process forproducing aromatic amines which can be carried out with unexpectedlylong on-line times and high selectivities.

These and other objects which will be apparent to those skilled in theart are accomplished by hydrogenation of a nitroaromatic compound in thegas phase in the presence of palladium-lead catalyst on a graphite orgraphitic carbon support and a large excess of hydrogen. Heat may beremoved from the catalyst bed by means of an optional heat carriersystem. Simple adiabatic reactor constructions which until now could notbe used for nitrobenzene hydrogenation are adequate. The continuouslyhigh selectivity of this process makes an elaborate working-up of thecondensed aromatic amine by distillation unnecessary.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to a process for the production of anaromatic amine represented by the formula ##STR1## in which R¹ and R²,independently of each other, represent hydrogen, a methyl group or anethyl group, and R¹ can also represent an amino group, by hydrogenationof a nitroaromatic compound represented by the formula ##STR2## in whichR² and R³, independently of each other, represent hydrogen, a methylgroup or an ethyl group,

and R³ may also represent a nitro group, with hydrogen on a stationarycatalyst in the gas phase. The catalyst is made up of palladium and leadon graphite or a graphite-containing coke support which has a BETsurface area of 0.2 to 10 m² /g. The catalyst has a palladium content,relative to the total weight of the catalyst, of from 0.001 to 7 wt. %,preferably from 0.1 to 6 wt. %, more preferably from 0.5 to 5 wt. %,most preferably from 1.5 to 4 wt. %, and a lead content of preferablyfrom 80 to 1000, most preferably from 100 to 300 equivalents of hydrogenper equivalent of nitro groups are fed to the catalyst.

The present invention also relates to a catalyst on a support ofgraphite or graphite-containing coke which has a BET surface area offrom 0.2 to 10 m² /g, a palladium content (applied by impregnation) offrom 0.001 to 7 wt. %, relative to the total weight of the catalyst, anda lead content of from 0.1 to 50 wt. %, preferably from 0.2 to 30 wt. %,relative to the palladium content.

Graphite-containing materials are used as support material for themanufacture of the catalysts. Examples of suitable graphites areelectrographite and cokes, such as needle coke or petroleum coke. Thesesupports generally have BET surface areas of from 0.2-10 m² /g.

The catalyst of the present invention may be prepared by applyingpalladium and lead separately or together to the support in from 1 to50, preferably from 2 to 30, most preferably from 4 to 10 impregnationsteps. Between each impregnation step, the catalyst support is dried ina hot gas stream, preferably an air or nitrogen stream. Betweenpalladium and lead impregnations, the palladium can be reduced, e.g.with hydrogen.

The catalysts of the present invention may be produced by applying thepalladium in the form of a suitable salt to the support which is in theform of tablets, spheres, granules, Raschig rings, Pall rings, cartwheels, or honeycomb structures of 1 to 30 mm diameters. When preparedby this method, several impregnation steps can be carried out withdrying after each application. The drying is carried out in an airstream at temperatures of from 30 to 140° C., preferably at 30 to 60°C., preferably at normal pressure. Aqueous and organic solvents as wellas mixtures thereof may be used for the impregnation of the support.Suitable solvents include: water, NH₃, simple alcohols, amines, ketones,esters, cyclic ethers, halogenated hydrocarbons and nitriles. Specificexamples of suitable organic solvents are methanol, ethanol, propanol,isopropanol, methylamine, ethylamine, isopropylamine, acetone, methylethyl ketone, dioxane, methylene chloride, acetonitrile and comparablesolvents. Suitable palladium salts include palladium chloride, palladiumnitrate, palladium acetylacetonate, palladium acetate and aminecomplexes of palladium. The catalysts are preferably prepared in theabsence of any halogen-containing solvent and metal salt.

Organic and inorganic lead compounds can be used to prepare thecatalysts of the present invention. Suitable organic lead compoundsinclude, for example, lead tetraalkyl compounds such as tetraethyl lead.Suitable lead salts include: lead chloride, lead nitrate and leadacetate. The catalyst is preferably prepared in the absence of anyhalogen-containing solvent or metal salt. After the impregnation and thefinal drying, the catalyst of the present invention is ready.

Before use, the catalyst may be activated by treating it with a hydrogenstream of from 1 to 10 bar at temperatures of from about 250 to about450° C., preferably from 300 to 400° C., for from about 1 to about 50hours, preferably from 5 to 30 hours.

The hydrogenation process of the present invention is generally carriedout at a pressure of from about 1 to about 30 bar, preferably from about1 to about 15 bar, most preferably from 1 to 7 bar.

Before the catalyst bed, the educt gas mixture containing nitroaromaticcompound and hydrogen has a temperature of from about 200 to about 400°C., preferably from about 230 to about 370° C., most preferably from 250to 350° C. The maximum catalyst temperature is 600° C., preferably 550°C., more preferably 500° C., most preferably 460° C.

The catalyst of the present invention may be used in reactors which donot have a system for heat removal.

The low residence times (i.e., high GHSVs) of the process of the presentinvention permit catalyst loadings of from 0.05 to 20 kg, preferablyfrom 0.1 to 10 kg of nitroaromatic compound per liter catalyst and hour.Such residence times are considered to be remarkable.

The process of the present invention is distinguished by the absence ofa catalyst startup phase and initial selectivities above 99.7%, whichafter a short time reach and exceed 99.9%.

Another important advantage of the process of the present invention isthe quantitative conversion of the nitroaromatic compound which,surprisingly and unexpectedly, is reached even at short residence timesor high GHSVs.

In the process of the present invention, the conversion of thenitroaromatic compound is generally greater than 99.95%, preferably morethan 99.99%, more preferably more than 99.995%, most preferably morethan 99.999%. It is, of course, possible to select process parameterswhich will result in lower conversions, if desired.

The catalysts of the present invention may be used in any desiredreactor having a stationary catalyst bed.

In one embodiment of the present invention, the catalyst is stationaryand is used in any one of the known adiabatic reactors. See, e.g.,Ullmann's Encyclopedia of Industrial Chemistry, 4th Edition, Vol. 3,pages 468-649; and Kirk-Othmer, Encyclopedia of Chemical Technology,Vol. 19 (1982), pages 880-914) for descriptions of such known reactors.The catalyst bed may, however, also be distributed over several reactorswhich are coupled in series or parallel. Examples of such reactors arethose known to be useful for the oxidation of methanol to formaldehyde.

The catalyst beds may be put on or between walls which are permeable togas. Care must, however, be taken to ensure that gas distribution issufficient. The catalyst may also be prepared and used on suitablepackings as support material instead of in beds.

Fresh nitroaromatic compound is metered into the circulating gas stream,which mainly consists of recycled and freshly added hydrogen, before thecatalyst bed. It is preferred that the nitroaromatic compound becompletely vaporized in the fresh hydrogen and then added as a gas tothe circulating gas stream. After passage through the catalyst bed, theproduct gas is cooled with steam recovery. Steam recovery may beaccomplished using any of the known heat exchangers. The product gas isthereafter cooled in order to remove aromatic amine and water ofreaction from the reaction mixture by condensation. The remainingcirculating gas, after diverting a small amount of gas for the removalof gaseous components in the circulating gas, is recycled. Beforerecycling, the circulating gas and fresh educt must be preheated inadmixture to inlet temperature.

The process of the present invention is particularly suitable forhydrogenating nitrobenzene and nitrotoluene.

The process of the present invention permits high catalyst loadings orGHSVs which exceed those of prior art processes. Moreover, the catalystsof the present invention make it possible to achieve selectivities above99.7% with complete conversion at the start of the reaction. Theseselectivities are the highest known to be achieved for the gas-phasehydrogenation of nitroaromatic compounds. These high selectivities makeit unnecessary to work-up the condensed aromatic amine by distillation.

The present invention is further illustrated but is not intended to belimited by the following examples in which all parts and percentages areparts by weight and percentages by weight, unless otherwise specified.

EXAMPLES

GHSV (Gas Hourly Space Velocity) indicates the hourly space velocity ofa gas under normal conditions or at the quoted pressure relative to theempty volume which the catalyst bed occupies.

Catalyst Preparation

Granular graphite EG 17 which is commercially available from theRingsdorff company, having a BET surface area of 0.4-0.8 m² /g was usedas support material for each of the catalysts used in these Examples.The grain size of this graphite was between 1 and 3 mm.

Other graphites and graphite-containing materials with small BET surfacearea when used as support materials for the catalysts of the presentinvention will, of course, produce similar results.

Each of the catalysts used in these Examples was prepared in thefollowing manner:

Granular graphite EG 17 (1-3 mm granules, tap density 650-1000 g/l) withan absorbency of 7 ml acetonitrile per 100 g support was charged to arotatable vessel. A solution of palladium acetate in acetonitrile wasadded during rotation. This mixture was kept moving until the solutionwas completely absorbed by the support. The solid was then dried for 5minutes in a rapidly rising stream of air at 40° C. The impregnating anddrying steps were repeated until the desired amount of palladium hadbeen applied.

Thereafter, impregnations and dryings were carried out in the same waywith aqueous lead acetate trihydrate solution until the desired amountof lead had been applied.

The dried catalyst was then activated in a hot hydrogen stream at normalpressure.

Example 1

Catalyst 1 (2% Pd, 0.5% Pb on EG 17) was prepared as described aboveusing:

2000 g support

9 impregnations, each with 9.25 g PdAc₂ in 140 g acetonitrile and

2 impregnations, each with 18 g PbAc₂.3H₂ O in 130 g water and

was then activated for 20 h at 370° C.

Example 2 (Comparative)

Catalyst 2 (comparative catalyst without lead, 2% Pd on EG 17) wasprepared using:

2000 g support and

9 impregnations, each with 9.25 g PdAc₂ in 140 g acetonitrile and

then activated for 20 h at 370° C.

Example 3

220 ml (219.0 g) of Catalyst 1 (prepared in Example 1), with 2.0% Pd and0.5% Pb were introduced, with a bed height of 180 mm, into a very wellinsulated reactor. The reactor was provided at its upper end with anevaporator and superheater. For the continuous discharge of the productgas, a well insulated tube was connected at the reactor outlet. Thiswell insulated tube led the product into a system of tube-bundle andcoiled-tube condensers for the purpose of condensation. The temperaturebefore, in and after the catalyst bed was measured by means of a slidingthermocouple. The catalyst was first treated at normal pressure in thereactor for 10 hours at 200° C. with hydrogen being supplied via anevaporator and a superheater. Thereafter, the hydrogen stream wasadjusted to 1620 l/h. At a starting temperature of T_(init) =202° C.,110 g/h nitrobenzene were metered by means of a metering pump via theevaporator-superheater into the hydrogen stream. This corresponded to amolar hydrogen/nitrobenzene ratio of 81/1 which at quantitativeconversion under adiabatic conditions would lead to a temperaturedifference between educt gas stream and product gas stream of about 200°C. After a few hours, a temperature profile established itself in thecatalyst bed. This profile corresponded to a heat loss via the reactorwall of approximately 10%. The rest of the heat of reaction left thecatalyst bed with the product gas mixture. The gas-chromatographicanalysis of the condensate yielded the results indicated in thefollowing Table. After 3000 h, the catalyst showed no signs whatever ofdeactivation.

GHSV=7460 h⁻¹

    ______________________________________    Running Catalyst Nbz*   Selectivity    time, h No.      ppm    %       T.sub.init ° C.                                           T.sub.max ° C.    ______________________________________     6      1        0      99.88   202    385    129     1        0      99.90   202    385    983     1        0      99.94   202    386    1804    1        0      99.93   202    383    2912    1        0      99.95   202    384    ______________________________________     *Nbz = Nitrobenzene

The continuously high selectivity exceeding 99.85% permitted thecondensed aniline to be used without further working-up. The purity ofthe product can optionally be raised further by partial condensationwhich will reduce the proportions of low- and high-boiling by-productsto less than 0.12%. It is therefore possible to avoid an expensiveworking-up by distillation.

Example 4 (Comparative Example with Lead-Free Catalyst)

The procedure of Example 3 was repeated using the same reactor with theexception that 220 ml (223 g) of Catalyst 2 with 2.0% Pd (prepared inExample 2) was employed. After hydrogen treatment, the metering ofnitrobenzene was begun at a starting temperature of T_(init) =201° C.After 40h, an aniline selectivity of 99.5% was obtained at quantitativeconversion. After 170 h, the selectivity to aniline had risen to morethan 99.6%. After 1000 h, the catalyst showed no sign of the start ofdeactivation. The results of this Example are presented in the followingTable.

GHSV=7460 h⁻¹

    ______________________________________    Running Catalyst Nbz*   Selectivity    time, h No.      ppm    %       T.sub.init ° C.                                           T.sub.max ° C.    ______________________________________     40     2        0      99.49   207    376    214     2        0      99.63   206    375    1004    2        0      99.73   207    377    ______________________________________     *Nbz = Nitrobenzene

Under otherwise identical conditions, use of Catalyst 2 resulted in morethan 400% more by-products than Catalyst 1.

Example 5 (Comparative)

The procedure of Example 3 was repeated using the same reactor, with theexception that 220 ml of a catalyst prepared in accordance with theprocedure described in Example 1 of DE 2,849,002, was used. Thiscatalyst was composed of 9 g Pd, 9 g V, and 3 g Pb on α-aluminum oxide(SPH 512 commercially available from the Rhone Poulenc Company). Afteractivation and hydrogenation under the same conditions used in Example3, the following results were obtained:

GHSV=7460 h⁻¹

    ______________________________________    Running   Nbz*   Selectivity    time, h   ppm    %           T.sub.init ° C.                                       T.sub.max ° C.    ______________________________________    3.5        0     98.8        199   383     90        0     99.1        195   372    160       100    99.6        200   375    ______________________________________     *Nbz = Nitrobenzene

In an oil-heated tubular reactor at the same nitrobenzene loading and ahydrogen/nitrobenzene ratio of 6/1 (GHSV=637 h⁻¹), this catalyst showeda life of about 1000 h and a selectivity averaging about 99.8% over theconversion cycle.

This catalyst is therefore unsuitable for operation with large hydrogenexcess and large nitrobenzene loading. The selectivities obtained wereclearly lower than those achieved with the process of the presentinvention.

The following Examples 6 and 7 were carried out using about 2 L ofCatalyst 1 at normal pressure in oil-heated heat-exchanger tubes of V2Ahaving a conventional length of about 300 cm and an internal diameter ofabout 3 cm.

The loading was 0.65 g/ml h and the temperature of the heat transfermedium was adjusted to 250° C.

Example 6

GHSV=9707 h⁻¹

H₂ /Nbz=81/1 Catalyst life>5000 h (no nitrobenzene breakthrough)

Example 7 (Comparative)

GHSV=2131 h⁻¹ (small hydrogen excess)

H₂ /Nbz=17/1 Catalyst life 108 h (nitrobenzene breakthrough>100 ppm)

Despite a clearly smaller gas loading or GHSV, Catalyst 1 deactivatedmore rapidly in Example 7 than in Example 6. It is therefore apparentthat below a critical hydrogen-to-nitrobenzene ratio, progressivecatalyst deactivation occurs.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A catalyst suitable for hydrogenating nitroaromatic compounds on a graphite or graphite-containing coke support having a BET surface area of from 0.2 to 10 m² /g which is impregnated with from 0.001 to 7% by weight palladium, based on total weight of the catalyst, and from 0.1 to 50% by weight lead, based on the weight of palladium.
 2. The catalyst of claim 1 in which the support is impregnated with the palladium by applying a palladium-containing solution to the support in from 1 to 50 impregnation stages and drying the catalyst support between each application with a hot gas stream.
 3. The catalyst of claim 1 in which the catalyst is activated in a hydrogen stream at 1 to 10 bar and at temperatures of from 250 to 450° C. 